“But wait”, you ask. “Since there’s already an existing patio or walkway in place, maybe we can save a few bucks by using it as a footing for a new flagstone overlay.” Well I’ll give you credit for some independent thinking.
But unfortunately, more often than not, it doesn’t work out.
Now before you go accusing me of up-selling again, let me tell you that I have done overlays on top of existing concrete in the past. And I wouldn’t necessarily be opposed to doing it again. But there’s a number of things that need to be looked at first.
1. First, the doorways. Does your patio door open directly onto the patio? Is there a step down to the patio or is the threshold set nearly even with the patio? Same goes for front doors and front entry landings. Keep in mind that a flagstone overlay typically will add a minimum of an inch-and-a-half of thickness to the surface to account for the thickness of the stones and the mortar bed. If you have an eight inch step from the doorway to the patio surface this isn’t necessarily an issue, but if it’s nearly even to begin with? Then it probably won’t work. You don’t want the patio surface to be higher than the interior floor. And you don’t want a small one-or-two inch step transitioning between surfaces. That’s what I like to call a tripper.
2. The layout. Assuming the interior entry points aren’t a problem, then the next thing you want to consider is the layout of the existing patio. Is the existing layout large enough for you? Is it a basic rectangle shape and you were hoping for something with a few sexy curves? If the existing layout is exactly what you were hoping to replicate, then green light it, game on.
But if you were hoping to keep the existing concrete as a partial footing and add on to make it larger or change the shape slightly, then you’re probably better off just tearing it out and starting anew. The cost saving of keeping that existing slab isn’t huge, and you don’t want a footing that isn’t one continuous slab. Even the slightest differential in
movement (concrete does move as the ground swells and contracts) can result in visible surface cracks or stones popping. Nothing you want to deal with and nothing I want my name attached to.
3. The integrity of the existing concrete. If you don’t like the existing concrete then there’s more than likely a problem with it. If it’s just a matter of not liking concrete, then hey, I’m with you. But is the surface starting to crumble or deteriorate? Then that’s going to affect the mortar bond to the overlay. Is it cracked? Well then the separate sections are likely to move independently of one another as the ground swells. As stated above, that can lead to visible cracks in the flagstone and/or stones becoming loose.
4. Does the existing concrete noticeably heave as the seasons change? I grew up in a house where our back patio heaved and settled about three to four inches every year. The slab remained intact, it just rose and settled with the seasons. That may sound a bit extreme, but even a small fraction of an inch heave is enough to pop a few stones or at least crack some mortar against the seams of the house. Why not alleviate the problem altogether by having a properly built footing installed to the proper depths and with the proper sub-prep. It’ll save everybody a few headaches down the road.
And maybe that existing concrete will work for you as a footing. I’ll look at it and let you know. But take a quick run-down of the above checklist first before you come to any assumptions about whether or not it can be re-used.